Method and system for fault protection using a linear feedback shift register

ABSTRACT

A method, system and apparatus for fault protection using a linear feedback shift register are presented. The system comprises a protected register containing a first portion for holding a value for a sequence of numbers and a second portion for other parameters. The system also comprises a linear feedback shift register circuit configured to write the sequence of numbers to the first portion of the protected register. If the written sequence of numbers of the protected register matches a reference sequence of numbers, a rewrite circuit is configured to write the second portion of the protected register.

RELATED APPLICATIONS

This application claims the benefit of priority to U.S. Provisional Application No. 60/950,217 filed Jul. 17, 2007, titled “LFSR Scheme for Laser Fault Protection in Optical Drives,” which is herein incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates generally to a fault protection mechanism. More specifically, the present disclosure relates to methods and systems for fault protection using a linear feedback shift register.

BACKGROUND

Modern devices can be susceptible to fault conditions, which may result in component and system damage. For example, during a drive operation of an optical drive such as a CD player, a CD/DVD combo player, a HD DVD player, or a Blu-ray player, the laser read power must be kept below a threshold, above which damage could occur to the laser. If the laser read power exceeds the threshold, the read laser may be damaged.

Typically, each device has built-in safety protection. In the above example, an optical drive typically includes an automatic power control loop. In addition, an optical drive may include a mechanism to shut down the read laser when the laser read power exceeds the threshold. The automatic power control loop can be disabled by an analog comparator circuit that continuously compares a forward sense voltage to the threshold. If the comparator detects a laser fault condition, the laser is automatically powered down, and an interrupt request is issued to notify device firmware of the fault condition.

However, a problem arises if a computer chip controlling the protection comparator erroneously sets the threshold to an inappropriate value. As a result, the device's built-in safety protection may not be invoked, even though the device is operating at an unsafe power level, because the power is under the erroneously-set threshold.

Moreover, the widespread use of firmware makes devices more susceptible to fault conditions. Firmware is a computer program embedded in a hardware component such as a microcontroller. A user may upload firmware onto existing hardware and update the firmware after the device is manufactured, either electronically, or by replacing a storage medium such as a socketed memory chip.

In optical drives, for example, built-in safety protection, such as an automatic power control loop, may typically be implemented in firmware. Therefore, it is conceivable that a laser fault condition could result if the firmware erroneously writes an inappropriate value to one or more of the register fields that determine the value of the current sent to the laser diode driver (LDD). One reason for this potential occurrence is a general firmware failure condition so severe as to require a power-on reset. Less severe firmware error conditions are also possible.

Notwithstanding any firmware errors, CPU failure, clocking errors, or other unexpected errors, the device must have a fault protection mechanism to protect the integrity of the threshold field. For example, in an optical drive, once the forward sense threshold value and other protected parameters have been set, the firmware cannot re-program them during normal drive operation, and the parameters may not be overwritten by other means. The parameters are thus protected in the sense that they may be programmed only once after each power-on operation.

Accordingly, it would be desirable to have additional methods and systems for fault protection using a linear feedback shift register.

SUMMARY

Consistent with principles of the invention, methods, systems and apparatuses for fault protection using a linear feedback shift register are presented. In one embodiment, a system comprises a protected register containing a first portion for holding a value for a sequence of numbers and a second portion for other parameters. The system also comprises a linear feedback shift register circuit configured to write the sequence of numbers to the first portion of the protected register. If the written sequence of numbers of the protected register matches a reference sequence of numbers, a rewrite circuit is configured to write the second portion of the protected register.

In another embodiment, a method for fault protection is provided. The method comprises performing a linear feedback shift register test using a protected register and writing at least one portion of the protected register if the linear feedback shift register test is successful. The linear feedback shift register test comprises generating a sequence of numbers, writing the generated sequence of numbers to the protected register, and providing a reference sequence of numbers. The outcome of the linear feedback shift register test is determined by comparing the generated sequence of numbers with the reference sequence of numbers. If the generated sequence of numbers matches the reference sequence of numbers, the linear feedback shift register test is considered successful.

In yet another embodiment, a method for changing a hardware parameter is provided. The method comprises generating a first sequence of numbers, initializing a reference sequence of numbers, writing the first sequence of numbers to a first portion of a protected register, and comparing the first sequence of numbers with the reference sequence of numbers. If the first sequence of numbers matches the reference sequence of numbers, write access to a second portion of the protected register is authorized.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a fault protection circuit, consistent with the present invention.

FIG. 2 is a flowchart showing an exemplary method for fault protection using a linear feedback shift register, consistent with the present invention.

FIG. 3 is a diagram showing a hardware implementation of a linear feedback shift register circuit, consistent with the present invention.

FIG. 4 is an exemplary software code showing software emulation of a linear feedback shift register circuit, consistent with the present invention.

FIG. 5 is a table showing an exemplary laser fault protection register, consistent with the present invention.

FIG. 6 is a table showing exemplary photodetector IC voltages, consistent with the present invention.

FIG. 7 is a table showing exemplary front monitor diode voltages, consistent with the present invention.

FIG. 8 is a flowchart showing another exemplary method for fault protection using a linear feedback shift register, consistent with the present invention.

FIG. 9 is a block diagram showing a fault protection system, consistent with the present invention.

FIG. 10 is a flowchart showing an exemplary method for changing a hardware parameter using a linear feedback shift register, consistent with the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

FIG. 1 is a block diagram showing a fault protection circuit 100. In this embodiment, fault protection circuit 100 includes a current input 110, a linear feedback shift register (LFSR) circuit 120, a protected register 130, a reference register 140, a rewrite circuit 150, a comparator 160 and a current output 170.

A LFSR is a circuit that generates a pseudorandom binary sequence corresponding to a sequenced binary bit stream using linear feedback. Consistent with the present invention, LFSRs and related logic may form a mechanism to restrict faulty access to protected hardware parameters.

In one embodiment, LFSR circuit 120 can be a hardware implementation of abstract algebra concepts. In particular, the input of a length-N shift register includes a linear combination of the current contents of the shift register. All arithmetic is performed modulo-2. In hardware, LFSR circuit 120 may be implemented using one or more XOR gates. In an example where N=4, only one XOR gate is required. As the LFSR continues to be updated, it produces a periodic sequence of N-bit values. If the LFSR feedback topology has been chosen properly, the period of this sequence will be 2^(N)−1. A sequence having this length (2^(N)−1) is called a maximal length sequence.

Although LFSR circuit 120 can be a hardware implementation, other implementations are possible. For example, LFSR circuit 120 may be implemented through software emulations in firmware.

The LFSR topology can be described by a polynomial. A polynomial that results in a maximal length sequence is called a primitive polynomial. LFSR circuit 120 may, for example, use a primitive polynomial where N=4: f(x)=x ⁴ +x+1  Equation (1).

The initial state of the LFSR is called the “seed.” Any N-bit seed value is permissible, except all zeros. Therefore, the LFSR may arbitrarily choose a seed value of: x=[x ₃ x ₂ x ₁ x ₀]=[0011]=3  Equation (2). The seed value is coded in hardware.

Once LFSR circuit 120 generates a sequence of numbers, starting with the seed value, LFSR circuit 120 writes the sequence of numbers, one at a time, to an LFSR field in protected register 130. The LFSR field in protected register 130, for example, may be the lower bits 3:0.

In one embodiment, the sequence of numbers in the LFSR field of protected register 130 may be compared with a reference sequence of numbers in an LFSR field of reference register 140. The reference sequence of numbers in reference register 140 may be initialized by the same LFSR algorithm as described above, either through a hardware LFSR circuit or an emulated firmware implementation.

The comparison between the sequence of numbers written in protected register 130 and the reference sequence of reference register 140 may be implemented in many ways. In one embodiment, for each of the numbers in the sequence of numbers generated by LFSR circuit 120 and written into LFSR field of protected register 130, that number may be compared with a number in the LFSR field of reference register 140. If the numbers match, LFSR circuit 120 will write the next number in the sequence into the LFSR field of protected register 130, thus overwriting the previous number. The number in the LFSR field of reference register 140 will also be advanced according to the reference sequence.

If LFSR circuit 120 writes the entire sequence of the numbers correctly, protected register 130 is unlocked, so that rewrite circuit 150 will be able to write the protected hardware parameters in protected register 130. The protected hardware parameters include, for example, a power threshold for read laser during drive operation.

At any time a mismatch occurs, however, the comparison process will be aborted, because a failure is considered to have occurred. Therefore, protected register 130 remains locked, and rewrite circuit 150 may not write the protected hardware parameters in protected register 130. Accordingly, the important hardware parameters, such as a read laser power threshold in an optical drive, are protected. For example, the hardware parameters may be programmed only once after each power-on operation. The important hardware parameters, however, may also be protected during normal operation under certain circumstances using the fault protection mechanism consistent with the present invention.

In one embodiment, LFSR circuit 120 is a firmware emulation of a hardware LFSR circuit, while reference register 140 is initialized through a hardware LFSR circuit. Both LFSR circuit 120 and rewrite circuit 150 provide write access to protected register 130 using the firmware. Other implementations of fault protection circuit 100 are also possible. In one example, LFSR circuit 120 may encompass the reference register 140 or store the reference sequence of numbers in other hardware locations without necessarily using reference register 140. In another example, the comparison of the sequences may be implemented by comparing more than one number at a time.

During normal operation, comparator 160 compares a characteristic of the system, such as the voltage or current with one of the protected hardware parameters and shuts down the system if the voltage or current exceeds the threshold in the protected hardware parameters. In one embodiment involving an optical pickup unit (OPU) architecture, comparator 160 may be a forward sense comparator that compares a forward sense voltage with a read laser power threshold.

FIG. 2 is a flowchart showing an exemplary method 200 for fault protection using a linear feedback shift register. In this embodiment, step 210 starts the process to detect any fault conditions. Step 210 may be triggered by a condition such as after each power-on operation. That is, the power-on reset line of the chip must be asserted and then deasserted.

A fault protection circuit then performs an LFSR test at step 220. In one embodiment, the fault protection circuit generates a sequence of numbers and writes the sequence of numbers, one at a time, to a protected register. The fault protection circuit compares the sequence being written to the protected register with a reference sequence of numbers in hardware. For example, for each of the numbers in the sequence of numbers in the protected register, that number may be compared with a number in the reference sequence of numbers. If the numbers match, the fault protection circuit will write the next number in the sequence to the protected register, and advance the state of the reference sequence of numbers. The fault protection circuit then compares the next number.

In step 230, the fault protection circuit determines whether the LFSR test was successful. If the fault protection circuit writes the entire sequence correctly, that is, if both sequences of numbers match in their entirety, the LFSR test is considered successful. If, however, at any time a mismatch occurs, the LFSR test has failed and the comparison process will be aborted.

Therefore, if the LFSR test is successful, the fault protection circuit performs step 240, where the protected register is unlocked and the protected hardware parameters in the protected register may be written. In one embodiment, after the protected hardware parameters have been set, the fault protection circuit may not re-program them during normal operation, and the values may not be overwritten by other means (step 250).

On the other hand, if the LFSR test fails, the process ends at step 250, where the protected register remains locked, and the protected hardware parameters may not be overwritten by other means.

In one embodiment, exemplary method 200 includes steps of initializing the reference sequence of numbers and storing the reference sequence in a reference register. In another embodiment, exemplary method 200 includes a step of using firmware to write the protected register.

In yet another embodiment, exemplary method 200 includes steps of generating a linear feedback shift register sequence of 4-bit values and/or writing a hardware parameter for a read laser power threshold in an optical drive such as a CD player, a CD/DVD combo player, a HD DVD player, and Blu-ray player. During normal drive operation, if a forward sense voltage exceeds the read laser power threshold, exemplary method 200 further includes a step of shutting down the optical drive.

FIG. 3 is a diagram showing an LFSR circuit 300 in a hardware implementation. In this embodiment, each number is a 4-bit value (N=4). The primitive polynomial in Equation (1) leads to the hardware structure in LFSR circuit 300.

LFSR circuit 300 includes shifters 310, 320, 330, and 340, corresponding to bits x₀, x₁, x₂, and x₃, and an XOR gate 350. In one embodiment, the seed is set to be x=[x₃x₂x₁x₀]=[0011]=3, although any 4-bit seed value is permissible except for all zeros. Accordingly, the periodic sequence of 4-bit values produced by LFSR circuit 300 is x={0011, 0111, 1111, 1110, 1101, 1010, 0101, 1011, 0110, 1100, 1001, 0010, 0100, 1000, 0001, . . . }. The period of this sequence is 15.

FIG. 4 shows an exemplary software code 400 providing software emulation of an LFSR circuit. Exemplary software code 400 emulates the hardware implementation of the above LFSR circuit 300 (see FIG. 3) in software. Other embodiments of software implementations of an LFSR circuit are possible, and exemplary software code 400 is provided here for illustrative purposes.

FIG. 5 is a table showing an exemplary laser fault protection register 500 in an OPU architecture. Register 500 may be an embodiment of protected register 130 in FIG. 1. Referring back to FIG. 5, register 500 includes fields 510 through 570: LFSR Value (field 510), Lock Indication (field 520), Forward Sense (FS) Threshold (field 530), Front Monitor Diode (FMD) Vref Select (field 540), Photodetector IC (PDIC) Vref Select (field 550), Deadman Counter Enable (field 560), and Reserved (field 570). The bit field and default value for each field are also shown in FIG. 5. For example, field 510 contains the LFSR value occupying the lower three bits of the register (3:0) with a default seed value of [0011]. The default values represent the initial state of laser fault protection register 500.

The descriptions of individual fields are explained next. LFSR Value field 510 may be the portion of the protected register to which LFSR circuit 120 (see FIG. 1) writes the sequence of numbers. Lock Indication field 520 is read only during normal operation and indicates whether the other portions (i.e., higher bits) of register 500 may be written.

To unlock the higher bits of register 500, the firmware must write the correct sequence of fifteen numbers to LFSR Value field 510. After the fifteenth correct value of the sequence is written, the supervisory logic unlocks the higher bits of register 500, allowing the desired parameter values to be written. If at any time an incorrect value is written, that is, the number does not match a corresponding number of a reference sequence, the hardware LFSR that regulates write access is reset and the register remains locked. The handshaking process may then be restarted without asserting the power-on reset line.

Once register 500 has been successfully written, no further writes are permitted unless the conditions are met. Register 500 may be read at any time, and a read operation does not alter the state of the hardware LFSR.

FS Threshold field 530 may be firmware-accessible. In one embodiment involving an OPU architecture, a transimpedance (IN) amplifier that converts a FMD photocurrent into a voltage is included on board the LDD IC. When the LDD IC is powered down, the FS signal is no longer actively driven. Even though there is no current flowing in the laser diode, the difference between the FS voltage and the reference voltage can become larger than the detection threshold, causing a false indication of a fault condition. For this reason, the FS voltage used by the forward sense comparator, such as comparator 160 (see FIG. 1), is referenced to ground, rather than to the reference voltage.

Therefore, this reference voltage may be protected based on the LFSR test mechanism for the LFSR Value. This may prevent any false fault detection that might otherwise occur as a result of an erroneous write to the reference voltage generator that is connected to the FS transimpedance amplifier within the OPU. This safety mechanism may also ensure that the fault detector signals when there is a true fault condition. Both the PDIC and FMD reference generators are protected in this way to ensure the integrity of FMD Vref Select field 540 and PDIC Vref Select field 550. Possibly, the Printed Circuit Board (PCB) designer may interchange the two reference generators.

Deadman Counter Enable field 560 has a default state of 0, indicating that the Deadman Counter is disabled. If field 560 has a state of 1, indicating that the Deadman Counter is enabled, the Deadman Counter may be adapted to count from an initial value, after the power-on reset line of the chip has been deasserted, to a final value to indicate that the system has failed and the chip should be shut down. In one embodiment, the Deadman Counter counts down from a pre-determined value to zero. Once it goes to zero, the system resets the chip. An LFSR circuit may also enable or disable the Deadman Counter using the Deadman Counter Enable field 560. If enabled, the firmware may reset the value of the Deadman Counter, so that the Deadman Counter does not go to zero unless a fatal system error has occurred. This way, the Deadman Counter will not be accidentally enabled or disabled. Also, the Deadman Counter can be dynamically disabled through the LFSR test mechanism during chip power-down mode to avoid the system being shut down.

In one embodiment, after power-on, but before the threshold register has been programmed, all laser power output is disabled and read-only Lock Indication field 510 is not set. When the higher bits of register 500 are programmed, the read-only Lock Indication field 510 is set to indicate success and becomes read only thereafter. After the higher bits of register 500 have been programmed, if the FS voltage exceeds the programmed FS Threshold value in field 530, the system is disabled immediately. This effectively forces the laser current to zero. In order to re-enable the laser, the power-on reset is required. After a power-on reset, the firmware must conduct the LFSR test, as discussed above. In addition, an interrupt is provided to the processor when laser power output has been disabled due to exceeding the threshold. The firmware can act accordingly by issuing a power-on test.

FIG. 6 is a table 600 showing exemplary photodetector IC voltages, while FIG. 7 is a table 700 showing exemplary front monitor diode voltages. In FIG. 6, table 600 includes a field 610 indicating register field values and a field 620 indicating PDIC reference voltages (Vref). For example, if PDIC Vref Select field 550 (see FIG. 5) has a value of 0, the PDIC reference voltage is 1.50 V. Similarly, in FIG. 7, table 700 includes a field 710 indicating register field values and a field 720 indicating FMD reference voltages. For example, if FMD Vref Select field 540 (see FIG. 5) has a value of 0, the FMD reference voltage is 1.50 V.

FIG. 8 is a flowchart showing another exemplary method 800 for fault protection using a linear feedback shift register. Method 800 starts at step 810, after the power-on reset line is asserted and then deasserted. The fault protection system next initializes a sequence of numbers with a seed value of [0011], using a firmware LFSR circuit, at step 820. The firmware then writes the sequence of numbers, one at a time, to register bits 3:0 (see the field 510 in FIG. 5) in the hardware, at step 830.

At step 840, the fault protection system checks whether the number written by the firmware matches a number from a reference sequence of numbers generated by a hardware LFSR circuit. If there is no match, the fault protection system returns to step 820 and initializes another sequence of numbers with a seed value of [0011], using a firmware LFSR circuit. If there is a match, however, the fault protection system will advance the firmware sequence to the next state at step 850. Also at step 850, the hardware reference sequence will advance automatically when a correct value is written to register bits 3:0 by the firmware.

Any attempts to write to register bits 14:5, such as fields 530 through 570 in FIG. 5, may not be accepted by the hardware until the fifteenth correct number has been written to the hardware by the firmware. The fault protection system continuously checks whether the firmware number equals the seed value [0011] at step 860 throughout the sequence. When there is no match, the fault protection system goes back to step 830 to advance to the next number in the firmware sequence. After the firmware has written the fifteenth correct numbers, the sequence starts again and the firmware number matches the seed value [0011]. This indicates the end of a successful LFSR test. At step 870, the fault protection system unlocks the upper bits of the protected register by setting the Lock Indication field 520 (see FIG. 5) to 1. As a result, the firmware may write the protected parameters, such as a read laser power threshold, in the upper bits of the protected register. Accordingly, during normal operation, the fault protection system may shut down the system if a FS voltage exceeds the read laser power threshold.

FIG. 9 is a block diagram showing a fault protection system 900, consistent with the present invention. Fault protection system 900 contains an LFSR circuit 920, a Deadman Counter enable circuit 940, a lock protection circuit 960, and a hardware parameter rewrite circuit 980.

LFSR circuit 920 comprises a firmware register 908, an LFSR hardware 910, an equal comparator 912, a comparator 914, an equal comparator 916, and an AND gate 918. Firmware register 908 is a protected register and may be read and written by the firmware. For illustrative purposes, the same firmware register 908 is presented multiple times, in circuits 940, 960, and 980. An embodiment of firmware register 908 is shown in FIG. 5.

Firmware register 908 has lower bits 3:0 for an LFSR value. Since the LFSR value has 4 bits, the period of the periodic sequence produced by LFSR circuit 920 is 15. In one embodiment, using the seed value is [0011], the periodic sequence is x={0011, 0111, 1111, 1110, 1101, 1010, 0101, 1011, 0110, 1100, 1001, 0010, 0100, 1000, 0001, . . . }.

After power-on, the firmware writes the first number of the LFSR sequence to the lower four bits of firmware register 908. LFSR hardware 910 calculates the next LFSR number. Equal comparator 912 compares the next LFSR number from LFSR hardware 910 with the next input number by the firmware. If the numbers match, comparator 914 allows the firmware to write the next input number to the lower four bits of firmware register 908, and the process continues through the fifteen numbers in the LFSR sequence. If at any time the numbers do not match, however, comparator 914 will generate an input to reset firmware register 908 and the process restarts.

In addition, equal comparator 916 continuously compares the lower four bits of firmware register 908 with the next to the last number in the LFSR sequence. If these two numbers do not match, AND gate 918 generates a negative output and the higher bits of the firmware register 908 remain unlocked and may not be rewritten by the firmware, as shown in circuits 940, 960, and 980. On the other hand, if these two numbers match, AND gate 918 will check the other input from equal comparator 912 to see if the last LFSR number matches the last input number by the firmware. If the last LFSR number also matches the last input number by the firmware, AND gate 918 generates a positive output to circuits 940, 960, and 980.

Deadman Counter enable circuit 940 includes an AND gate 944 that receives one input from AND gate 918 and the other from a firmware write enable signal. If both of them are positive, AND gate 944 will enable write access to Deadman Counter enable field 560 (see FIG. 5). A firmware write input may then reset the Deadman Counter and the system. If any of the input of AND gate 918 is negative, the Deadman Counter may not be reset.

Similarly, lock protection circuit 960 includes an AND gate 964 that receives one input from AND gate 918 and the other from a firmware write enable signal. If both of them are positive, AND gate 964 will enable write access to Lock Indication field 520 (see FIG. 5). A firmware write input may write to the upper bits of firmware register 908, as the upper bits are unlocked. If any of the input of AND gate 918 is negative, firmware register 908 remains locked.

Hardware parameter rewrite circuit 980 also includes an AND gate 984 that receives one input from AND gate 918, one from a firmware write enable signal, and an inverse input from Lock Indication field 520 of firmware register 908 (see FIG. 5). If all three of the inputs are positive, AND gate 984 will enable write access to the laser protection threshold (e.g., FS Threshold field 520), FMD Vref Select field 540, and PDIC Vref Select field 550, as shown in FIG. 5). Either FMD Vref or PDIC Vref may be used on-chip and off-chip as a reference voltage.

Referring back to FIG. 9, after the laser protection threshold is set, a threshold comparator 986 will compare a reference voltage with the laser protection threshold and disable the laser if the reference voltage exceeds the laser protection threshold.

FIG. 10 is a flowchart showing an exemplary method 1000 for changing a hardware parameter using a linear feedback shift register, consistent with the present invention. After starting at step 1010, a fault protection system generates a first sequence of numbers at step 1020, using, for example, a firmware LFSR circuit. The fault protection system then initializes a reference sequence of numbers using, for example, a hardware LFSR circuit at step 1030. At step 1040, the firmware writes the first sequence of numbers to a first portion of a protected register, such as the lower four bits of the protected register.

The fault protection system then compares the first sequence of numbers with the reference sequence of numbers, one at a time, at step 1050. If the comparison is successful for all the numbers in the sequence, the protected register is unlocked and the firmware may write the higher bits of the protected register at step 1060. If, however, the comparison fails for any of the numbers in the sequence, the protected register remains unlocked and the process ends at step 1070. The fault protection system may also restart the process.

In one embodiment, exemplary method 1000 includes performing a power-on test before starting the process at step 1010. In another embodiment, exemplary method 1000 includes storing the reference sequence of numbers in a reference register. The numbers in the sequence may have 4-bit values. In yet another embodiment, exemplary method 1000 includes writing a hardware parameter for a read laser power threshold in an optical drive such as a CD player, a CD/DVD combo player, a HD DVD player or a Blu-ray player. Exemplary method 1000 may also include shutting down the optical drive if a forward sense voltage exceeds the read laser power threshold.

Notably, the systems and methods disclosed herein are not inherently related to any particular computer or other apparatus, and may be implemented by a suitable combination of hardware, software, and/or firmware. Software implementations may include one or more computer programs. A computer program is a set of instructions readable and executable by a processor and can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program can be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network. Software may also be implemented as a computer program product, i.e., one or more computer programs tangibly embodied in an information carrier, e.g., in a machine-readable storage device, for execution by, or to control the operation of, data processing apparatus, e.g., a programmable processor, a computer, or multiple computers.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims. 

What is claimed is:
 1. A system for fault protection, the system comprising: a protected register comprising a first portion for holding a value for a sequence of numbers and a second portion for other parameters; a linear feedback shift register circuit configured to write the sequence of numbers to the first portion of the protected register; and a rewrite circuit configured to write the second portion of the protected register if the written sequence of numbers of the protected register matches a reference sequence of numbers.
 2. The system of claim 1, further comprising: a reference register configured to provide the reference sequence of numbers.
 3. The system of claim 2, wherein the reference register is implemented in hardware.
 4. The system of claim 1, wherein the linear feedback shift register circuit comprises firmware for write access to the protected register.
 5. The system of claim 1, wherein the rewrite circuit comprises firmware for write access to the protected register.
 6. The system of claim 1, wherein the sequence of numbers is a linear feedback shift register sequence of 4-bit values.
 7. The system of claim 1, wherein the second portion of the protected register contains a hardware parameter for a read laser power threshold in an optical drive.
 8. The system of claim 7, further comprising: a circuit for shutting down the optical drive if a forward sense voltage exceeds the read laser power threshold.
 9. A method for fault protection, comprising: performing a linear feedback shift register test using a protected register; and writing at least one portion of the protected register if the linear feedback shift register test is successful, wherein performing the linear feedback shift register test comprises: generating a sequence of numbers; writing the generated sequence of numbers to the protected register; providing a reference sequence of numbers; and comparing the generated sequence of numbers with the reference sequence of numbers to determine whether the linear feedback shift register test is successful.
 10. The method of claim 9, further comprising: initializing the reference sequence of numbers; and storing the reference sequence of numbers in a reference register.
 11. The method of claim 9, further comprising: using firmware to write the protected register.
 12. The method of claim 9, wherein generating a sequence of numbers comprises: generating a linear feedback shift register sequence of 4-bit values.
 13. The method of claim 9, wherein writing at least one portion of the protected register comprises: writing a hardware parameter for a read laser power threshold in an optical drive.
 14. The method of claim 13, further comprising: shutting down the optical drive if a forward sense voltage exceeds the read laser power threshold. 